1. Field of the Invention
The present invention relates to an anti-skid braking system for motor vehicles, adapted to apply brake to the vehicle, while preventing the wheels from skidding or locking on the road surface.
2. Discussion of the Prior Art
An anti-skid braking system is employed for an automotive vehicle, for braking the vehicle with a minimum braking distance, and without deteriorating the straight-line running stability of the vehicle. Usually, this type of braking system includes hydraulically operated wheel brakes for restraining a rotating movement of the vehicle wheels, actuators for regulating the brake fluid pressures applied to the wheel brakes, speed sensors for detecting the rotating speeds of the wheels, and a controller for presuming amounts of slip of the wheels based on the speeds detected by the speed sensors, and controlling the actuators. Generally, the controller operates to presume that an actual running speed of the vehicle is represented by the highest one of the speeds of the wheels. After the deceleration rate (i.e., deceleration value) of the highest speed has exceeded a preset upper limit, the controller fixes the deceleration of the vehicle at the preset upper limit, presumes the actual vehicle speed based on the upper limit, and calculates a slip amount of each wheel based on a difference between the presumed vehicle speed and the detected speed of the wheel.
While it is possible that the deceleration rate of the vehicle is fixed at an upper limit which is considered a maximum rate for the specific vehicle, the upper limit is actually reduced as the coefficient of friction of the road surface is lowered. As one solution to deal with this matter, laid-open Publication No. 58-26661 (published on Feb. 17, 1983) of Japanese Patent Application proposes to change the upper limit of the vehicle deceleration rate, depending upon the specific actual braking conditions. Described more particularly, when the deceleration rate of a highest one of the speeds of a plurality of wheels has exceeded a preset upper limit, the highest speed at that moment is stored in a memory. Subsequently, the speed of any one of the partly slipping wheels comes up to a peak level due to a decrease in the braking pressure applied to that wheel. The deceleration rate of the vehicle at this point of time is presumed based on a gradient of a straight line which connects the speed level stored in the memory, and the recovered peak level at the above-indicated point of time. When it later becomes necessary to fix the deceleration rate at an upper limit, the previously presumed deceleration rate is used as the upper limit. The proposed arrangement permits more accurate presumption of the actual running speed of the vehicle, for improved adequacy of controlling the anti-skid braking system.
According to the above proposed arrangement, each decleration rate presumed based on the gradient obtained in the manner described above is used as the upper limit, only at a later time when the deceleration rate of a highest one of the speeds of the wheels will exceed the upper limit in the next cycle of control. Namely, the presumed upper limit obtained in the preceding cycle is used when the current cycle indicates the necessity of fixing the deceleration rate at a suitable upper limit. Therefore, in the first cycle of control, an upper limit of the deceleration rate has not yet been presumed. This means that the proposed arrangement must employ as an upper limit in the first control cycle, a maximum deceleration rate that is considered to occur on a specific vehicle, as in the conventional arrangement. Consequently, the presumed running speed of the vehicle may considerably deviate from the actual running speed, if the employed maximum deceleration rate is excessively different from the actual rate, since the employed maximum rate is not based on such an extraordinary low friction coefficient of the road surface. Thus, the accuracy of control of the proposed anti-skid braking system is accordingly deteriorated. This is a problem remaining in the proposed conventional system.